High peak power laser cavity and assembly of several such cavities

ABSTRACT

A high peak power optical rest and combination of plural of the resonators, particularly to excite a light generator in the extreme ultraviolet. An optical resonator with a solid state amplifying medium is pulsed and pumped by diodes operating continuously. The optical resonator includes at least two laser rods, at least one mechanism to trigger optical pulses, the triggering mechanism located in a part of a cavity in which the laser beam diverges least, and first and second mirrors that delimit the cavity, the first mirror being highly reflecting and the second mirror being partly reflecting.

TECHNICAL DOMAIN

This invention relates to a high peak power optical resonator with ahigh mean power and a high recurrence rate, with minimum cost andcomplexity. It also relates to the combination of several of theseresonators, particularly to excite a light generator in the extremeultraviolet.

The invention is thus more particularly applicable to light generationin the extreme ultraviolet range.

Radiation within this range that is also called “EUV radiation” haswavelengths varying from 8 nanometres to 25 nanometres.

EUV radiation that can be obtained by making light pulses generated withthe device according to the invention interact with an appropriatetarget has many applications, particularly in the science of materials,microscopy and more particularly microlithography to make very largescale integrated circuits. For very large-scale integrated circuits, itis particularly advantageous to have a high recurrence rate, which isvery difficult to obtain for high peak power lasers.

The invention is applicable to any domain that requires an excitationlaser of the type necessary in microlithography.

STATE OF PRIOR ART

EUV lithography is necessary in microelectronics to make integratedcircuits with dimensions of less than 0.1 micrometers. Several sourcesof the EUV radiation use a plasma generated using a laser.

In particular, it is required to generate ultraviolet radiation with awavelength equal to about 13 nm by exciting a xenon jet with an intenselaser source.

Three conditions must be combined for this laser source to beeconomically satisfactory:

-   -   the peak power of the laser light must be very high (of the        order of 10¹¹ W/cm²) in order to create a sufficiently emissive        plasma around 13 nm,    -   the repetition rate must be high (several kilohertz) to make as        many semiconductor wafers as possible per hour, and    -   the laser source must be simple, it must have a reasonable        investment cost and a low operating cost.

Therefore, a laser generating a high peak illumination must be availableto create the plasma.

This is done using a pulse laser, for example outputting an energy ofthe order of 300 mJ per pulse or more.

Note that the invention can for example make use of YAG lasers dopedwith neodyme, and many developments have been made in many industrialfields for these lasers. However, other solid-state lasers, in otherwords lasers for which the amplifying medium is solid, can be used inthis invention.

We will discuss this point in more detail later.

It is known how to use pumping by laser diodes in order to obtain a verygood energy stability in each firing.

Furthermore, it is known how to use pulse diodes to obtain the peakpower necessary for generation of EUV radiation to be used forphotolithography.

The following document provides further information about this subject:

[1] Article by H. Rieger et al., High brightness and power Nd:YAG laser,Advanced solid-state lasers, 1999, Boston Mass., p. 49 to 53.

This document divulges a device for photolithography, generating highpeak amplitude laser pulses at a relatively low recurrence rate.

It is also known how to use an oscillator and amplifiers to obtain thenecessary peak power. This results in a complex and expensive laser.

The following document provides further information about this subject:

[2] Article by G. Holleman et al., Modeling high brightness kWsolid-state lasers, SPIE Vol. 2989, p. 15 to 22.

This document mentions two needs for power lasers corresponding to twoopposite technologies:

-   -   firstly, welding, machining and material treatment applications        that require lasers emitting long pulses obtained by very simple        technologies and,    -   secondly, photolithography applications that require short        pulses at a high rate if possible, obtained by a very        sophisticated and expensive technology, in particular using two        optical amplification stages.

Refer also to the following document, that describes a high peak powerlaser device:

[3] Article by G. Kubiak et al., Scale-up of a cluster jet laser plasmasource for extreme Ultraviolet lithography, SPIE Vol. 3676, p. 669 to678.

The device described in this document [3] uses YAG lasers doped withnoedyme, pumped by pulsed diodes as in the rest of prior art related tophotolithography. It also uses complex and expensive optical amplifiers.Furthermore, the target recurrent rate in this document [3] is 6 kHz,for a pulse energy of 280 mJ.

An improved version of this laser is described in document [6] discussedbelow.

Refer also to the following document:

[4] Article by H. Rieger et al., High brightness and power Nd:YAG Laser,OSA trends in Optics and Photonics, Vol. 26, from the topical MeetingJan. 31, Feb. 3, 1999 in Boston, Optical Society of America, p. 49 to53.

-   -   which briefly describes a device with a very low power master        oscillator outputting 1 mJ pulses at a maximum frequency of 1        kHz (therefore with an average power equal to not more than 1        W), followed by a complex and expensive amplification system.        The essential part of this document consists of studying the        degradation of the quality of the beam in this amplification        system. The device described has performances well below the        performances required for an EUV source to be used in        microlithography, both in terms of the average power and of the        repetition frequency.

The characteristics required for a laser device that could excite anintense EUV radiation source compatible with the needs of thesemiconductors industry have been standardised on a world scale in theform of a specification, and many attempts have been made to satisfythis specification.

However, up to now, all these attempts have failed.

The strict constraints in the specification obviously include theability to generate high peak intensities with a very high recurrencerate. But there is also the need to obtain a good quality beam,characterised by the lowest possible value of the magnitude M² that isdefined as being the product of the beam diameter, and the angle of itsdivergence and a constant.

The theoretical lower limit of M² is equal to 1, but as the laser powerincreases, the value of M² increases. It typically reaches several tenswith a YAG laser doped with neodyme, also called an Nd:YAG laser.

The specification mentioned above imposes M²≦10.

Other more recent documents divulge devices intended to satisfy thisspecification:

[5] Article by K. Nicklaus et al., Industry-Laser Based Short PulseDiode Pumped Solid State Power Amplifier With kW Average Power, OSATrends in Optics and Photonics, Vol. 50, Advanced Solid-State Lasers,Christopher Marshall, ed., Optical Society of America, 2001, p. 388 to391,

-   -   which describes a device in which the optical resonator outputs        4 mJ pulses at 2 kHz (or 8 mJ pulses at 1 kHz) to a set of two        double passage preamplifiers. The return path of the beam is        deflected by a polarising cube to a line of two amplifiers,        whose output delivers 76 mJ (the structure of such a device is        called a MOPA: Master Oscillator Power Amplifier).

[6] Article by D. A. Tichenor et al., EUV Engineering Test Stand,Emerging Lithographic Technologies IV, Elisabeth A., Dobisz, Editor,Proceedings of SPIE Vol. 3997 (2000), p. 48 to 69.

This article describes a laser installation using three identicalmodules put in parallel, each of these modules being composed of thelaser made by the TRW Company and described in the following document:

[7] Active Tracker Laser (ATLAS), Randall St. Pierre et al., OSA TOPS,Vol. 10, Advanced Solid State Lasers, 1997, p. 288 to 291.

The Nd:YAG solid-state optical resonator described in document [7]outputs pulses of 1.6 mJ at 2.5 kHz, which are amplified in a doublepass structure producing output pulses of 276 mJ. A slightly earlierversion of this TRW laser was described in document [3].

According to documents [5] and [6], light pulses are generated in abasic laser containing a very small low energy oscillator (less than 10mJ per pulse) with low average power (less than 15 W), and they areamplified by many passes in rod or plate amplifier stages, in order toobtain a high power with a low value of M² and very short pulses.

The problem then arises that when the incident light power is lowcompared with the saturation fluence of the laser rod used (andparticularly for incident fluences less than 200 mJ/cm² for the Nd:YAG),the amplification provided by the rod is very weak. A large number ofamplifying rods which are extremely expensive are then necessary,together with several tens of diodes which are also very expensive, andthe energy efficiency of the final result is very low.

In order to limit the installation cost, there are usually two passesthrough the first stage(s) (forward-return path, which is why it iscalled the double pass amplifier), which makes it necessary to work witha polarised beam and to use a polariser (for example a polariser cube)so that the return path does not return onto the oscillator but isswitched along another optical path, along which the amplification willbe continued.

This need to polarise the beam introduces an additional problem in thecase in which the double pass amplification uses an isotropic materialfor example such as the Nd:YAG or the Yb:YAG, as the amplifying rod. Theisotropy of this type of material is modified at the time of pumping,which degrades the polarisation of the incident beam.

Thus, if complex devices were not installed to limit this phenomenon,polarisation would not be sufficiently maintained and a large part ofthe beam energy (about 25% for Nd:YAG) would be lost when the returnbeam entered the polariser, and this could destroy the oscillator.

These complex devices, in other words essentially associations ofpolarisation rotators and judiciously placed phase plates, limit thepower of the beam returning to the oscillator to a low value (about 2.4%for Nd:YAG).

Thus, in order to solve the problem of obtaining a laser device capableof exciting an intense EUV radiation source compatible with the needs ofthe semiconductors industry, the authors of document [5] and also theauthors of documents [6] and [7] generated the most perfect possiblepulses with very low power, and then multiplied the number of amplifiersand concentrated all their efforts on research for means to limitdepolarisation losses in these amplifiers.

This method leads to complex and expensive devices with a low energyefficiency. Furthermore, for the devices described in documents [5] and[7], the main elements were placed in series. Thus, any failure ofeither of them will affect the entire device.

Another method was proposed in the following document:

[8] Compact 300-W diode-pumped oscillator with 500 kW pulse peak powerand external frequency doubling, Oliver Melh et al., OSA trends inOptics and Photonics (TOPS), Vol. 56, Conference on Lasers andElectro-Optics (CLEO 2001, May 6-11 2001, Technical Digest, pp. 421-422.

This document describes an Nd:YAG laser comprising two Nd:YAG rods, apolarisation rotator between these rods, two acousto-optical modulatorsone on each side of the two rods and a divergent lens between eachmodulator and the corresponding rod, all within an optical resonator.

The average output power of the optical resonator is 260 W, and therecurrence rate is 10 kHz.

However, the implementation described in this document ignores animportant problem related to light pulse triggering (Q-switching)devices, particularly acousto-optical Q-switch devices used in the laserdescribed in this document; the problem is that their operation dependson the divergence of the laser beam.

Acousto-optic triggers (Q-switches) essentially comprise anacousto-optic crystal and a control device and operate as follows:

When it receives an electrical signal, the control device emits a radiofrequency excitation wave in the crystal, which generates a Bragggrating in this crystal. When there is no excitation, this crystalallows incident rays to pass, which under nominal operating conditionsdo not arrive along the normal to the entry face of the crystal, butmake a Bragg angle with it.

When the control is activated, the radio frequency wave generates theBragg grating that then deflects the incident light rays; the deflectionangle is sufficient so that these rays leave the optical resonator,which corresponds to cutting off the beam laser.

When light rays arrive on the entry face to the crystal at an angle notequal to the Bragg angle, they are no longer suitably deflected,particularly if they shift by an angle close to a limiting or criticalangle, or greater than this limiting angle.

The value of this limiting angle is practically the same as the value ofthe angle between the directions of the first and second order beamsdiffracted by the Bragg grating formed in this crystal when it isexcited (typically about 4 mrad).

Rays with an angle of incidence close to this angle are not correctlyintercepted when the crystal is excited. Rays for which the incidenceexceeds this angle are no longer suitably deflected, but also theyreturn towards the central part of the optical resonator since theirincidence is within the angular acceptance of this cavity.

They then make the cavity emit in an unwanted manner, which generatesemission of some continuous laser light power at the output. Operationbecomes erratic, and pulses with an unstable amplitude and durationsuperpose on this continuous laser emission at the output from theresonator.

For the same beam divergence, the instability increases as the pulsepower required from the cavity increases.

PRESENTATION OF THE INVENTION

The purpose of this invention is to solve the problems inherent to MOPAstructures used in embodiments described in documents [5] to [7] andproblems inherent to structures with an oscillator outputting a highpower but for which the stability is affected by limitations toacousto-optical triggers (Q-switches), as in the embodiment described indocument [8].

The invention is intended to solve them using an optical resonator witha high peak power and high recurrence rate, and by the association ofthis cavity with other identical cavities to form a laser device toachieve higher peak power performances than are possible with devicesdisclosed by documents [5] to [8], while being less complex, lessexpensive and with more reliable operation.

Note also that the laser devices disclosed by document [5] are designedto obtain short duration pulses from 5 ns to 20 ns, which personsskilled in the art consider as being favourable to obtaining a veryemissive plasma.

Specifically, the purpose of this invention is an optical resonator witha solid state amplifying medium, this optical resonator being pulsed andpumped by diodes operating continuously, and characterised in that itcomprises:

-   -   at least two laser rods,    -   at least one means of triggering light pulses, this triggering        means being located in the part of the resonator in which the        laser beam generated by the resonator diverges least, and    -   two mirrors that delimit this resonator, one being highly        reflecting and the other being partly reflecting.

In the simplest case of a resonator with two laser rods, the part of theresonator in which the laser diverges least is the part located betweenthe two rods.

At the opposite side, the parts of the resonator located outside therods between one of the rods and one of the mirrors of the resonator,are the parts in which the beam diverges most.

The implementation described in document [8] places the light pulsetriggering means in these parts, which makes them subject to thedysfunctions described for the state of prior art.

If the laser rods are made from an isotropic material such as Nd:YAG orYb:YAG, it is necessary to add a polarisation rotation means on the pathof the beam in each of the spaces formed by two successive rods, thisrotation preferably being 90°, in order to obtain the beam qualityspecified for the microlithography industry.

Advantageously, the slight convergence produced by some laser rods, andparticularly Nd YAG, is corrected by placing, on the beam path, a lenswith an opposite effect on convergence, in the middle of each intervalbetween two adjacent rods.

According to one preferred embodiment of the device according to theinvention, the laser material from which the laser rods are made ischosen in the group comprising Nd YAG, Nd:YLF, Nd:YALO, Yb YAG, Nd:ScO₃and Yb:Y₂O₃.

Preferably, the resonator according to the invention comprises two rodsmade of a laser material, preferably substantially identical,polarisation rotation means placed in the resonator between these tworods, and two means of triggering pulses placed between the two rods oneach side of the polarisation rotation means.

Preferably, the triggering means are of the acousto-optical type.

According to one variant embodiment, the optical resonator according theinvention could be associated with one or several single pass laseramplifiers pumped by diodes, the rod for each amplifier being activatedover its entire length at or above the saturation fluence of the rodmaterial.

Preferably, this fluence is equal to at least three times the materialsaturation fluence.

Functionally, the optical resonator is characterised by its capabilityof producing a stable output with a high fluence without it beingnecessary to make the beam that it generates converge. It can keep theparallelism of this beam and reach or exceed this saturation fluenceover the entire length of the rod.

In the preferred application that will be described in detail later,this fluence is equal to about ten times the material saturationfluence.

The invention also relates to the association of at least three opticalresonators of the type described above, arranged in parallel but forwhich the beams that they generates are directed towards the sametarget.

The laser device resulting from this combination of these cavities ischaracterised in that it comprises:

-   -   at least three pulsed optical resonators with a solid state        amplifying medium, these resonators complying with the optical        resonator according to the invention, and    -   means for transferring these light pulses to substantially the        same location on a target and at substantially the same time at        this location,    -   and in that the device also comprises means of controlling the        pulsed optical resonators, these control means being designed so        that all pulses reach the target at practically the required        instant with a precision better than 5 ns, and preferably better        than 1 ns.

According to one variant, the optical resonators are associated with oneor several single pass amplifiers.

According to a particular embodiment of the device according to theinvention, the triggering means for each pulsed optical resonatorcomprise two triggers (Q-switches) placed in this resonator, on eachside of the polarisation rotation means, between these means and therods made of a laser material.

According to one particular embodiment of the invention, the means ofsending light pulses comprise means of sending these light pulses ontothe target along the same path.

According to one particular embodiment of the device according to theinvention, this device also comprises means of modifying the spatialdistribution of the light pulse resulting from the addition of lightpulses output by the optical resonators.

According to another particular embodiment, the means of controlling theoptical resonators are also capable of modifying the time distributionof the light pulse resulting from the addition of light pulses suppliedby the optical resonators, in order to create composite pulses.

According to one particular embodiment of the invention, the profile ofeach composite pulse comprises a first plasma ignition pulse that willbe created by interaction of the light pulses with the target, a timeinterval in which the light energy output by the laser is minimum duringplasma growth, and then a second pulse composed of several elementarypulses according to a sequence that depends on plasma growth.

If composite pulses are created, the device according to the inventionis preferably capable of sending a first highly focused beam onto thetarget, and then applying the remainder of the light energy onto thetarget with broader focusing.

The target on which light pulses emitted by the optical resonators inthe device according to the invention are emitted, may be designed tooutput light in the extreme ultraviolet domain by interaction with theselight pulses.

However, this invention is not limited to obtaining EUV radiation. It isapplicable to any domain in which high peak power laser beams directedonto a target are necessary.

A spatial superposition is used in this invention, and in a particularembodiment a time sequence is used.

“Spatial superposition” means superposition of a plurality of laserbeams substantially at the same location of the target, andsubstantially at the same time.

“Substantially at the same time” means that the time differences betweenthe various elementary pulses supplied by the different opticalresonators in the laser device are small compared with the recurrenceperiod of these optical resonators. This superposition makes it possibleto multiply the energy per pulse and peak powers.

As will be seen later, versatility can be obtained with superposition ofthe laser beams at almost the same location and almost the same time.This versatility makes it possible for the resulting laser beam to beadapted to requirements of the plasma.

In this invention, points (a) to (c) described below are important.

a) Spatial Superposition

Spatial superposition increases the peak power and gives broad freedomto modify the spatial distribution of the light pulse resulting from theaddition of the elementary light pulses emitted by the opticalresonators.

For example, the use of one light pulse more focused than the others asimplemented in one preferred embodiment of the invention, can give agreater local illumination as shown diagrammatically in FIGS. 1 and 2,in which only two beams are shown to simplify the drawings.

A first light beam F1 and a second light beam F2 are shown in asectional view in FIG. 1, in a plane defined by two perpendicular axesOx and Oy, the axis common to the two beams being the Oy axis.

The two beams have approximately the same symmetry of revolution aboutthis Oy axis and are focused close to the point O, substantially in theobservation plane defined by the Oy axis and by an axis perpendicular tothe Ox and Oy axes and that passes through the point O.

The focussings of the two beams are different, the first beam F1 beingmore tightly focused than the second beam F2.

FIG. 2 shows variations of the illumination I in the observation planeas a function of the abscissa x along the Ox axis.

If beam F1 is five times more focused than beam F2, the illuminationproduced by this beam F1 on the Oy axis is twenty five times greaterthan the illumination produced by the beam F1 when the two beams havethe same power. But note that with this invention, beams with identicalpowers could be used, or on the other hand the beams could havedifferent powers or very different powers from each other.

This “spatial superposition” with several beams on the same target atthe same time enables an offset of the times of pulses of eachelementary optical resonator, on a smaller time scale.

-   -   b) Sequencing in Time of the Different Laser Pulses (“Composite”        Pulses)

Pulse bursts can be created in which time offsets between two pulsesfrom two elementary optical resonators are very small compared with therecurrence time between two bursts. These types of bursts may beconsidered as being composite pulses.

A prepulse may also be created by a time offset of these light pulses.

Further information about this subject is given in the followingdocument that mentions the possibility of creating a charged prepulsefor ignition of the plasma:

[9] Article by M. Berglund et al., Ultraviolet prepulse for enhancedX-ray emission and brightness from droplet-target laser plasma, AppliedPhysics Letters, vol. 69, 1996, page 1683.

The invention preferably uses this sequence in time for the variouslaser pulses.

For example, it can be used to obtain the sequencing described below.

A first pulse highly focused on the target (for example this pulse beingof the type of beam F1 in FIG. 1) ignites a plasma, and then while theplasma is growing, the target is subjected to minimum or zeroillumination, and when the plasma reaches the diameter of the beam F2, amaximum light power is applied to the target. It is then advantageous ifthe energy dedicated to the first pulse is lower than the energydedicated to the remainder of the composite pulse as shown in FIG. 3.

In FIG. 3, the amplitudes A of the light pulses are shown as a functionof time t. It shows an example of a composite pulse 11. This compositepulse comprises a prepulse 12 followed by a first set of simultaneouselementary pulses 13, separated from the prepulse by a time T necessaryfor growth of the plasma, and then a second set of elementarysimultaneous pulses 14 following the first set.

-   -   c) Use of Continuous Diodes for Pumping the Laser Material

If an optical resonator using a YAG material doped with neodyme is usedwith continuous pumping, the life of the upper level of the opticalresonator that is close to 250 microseconds makes it necessary to workat a rate of more than 5 kHz to actually extract the deposited lightpower.

Unlike prior art, this invention can be used to obtain high peak powers,by associating an unfavourable point for this peak power (point c) and afavourable point (point a) with a weight that becomes greater as thenumber of elementary optical resonators is increased.

Point (b) is simply one possible way of adapting the invention to itsapplications as well as possible.

For an application to microlithography, this possibility enables thebehaviour of the EUV source pumped by the laser device to be optimisedto suit other plasma requirements.

However, in the current state of the art, it is considered preferable tomake all pulses arrive at the same time, within 5 ns, or even betterwithin 1 ns.

In this invention, points (a), (b) and (c) can all be used at the sametime, and this combination of favourable and unfavourable points forobtaining high peak powers is contrary to prior art.

Advantages of this invention, apart from the generation of high powerand high speed laser pulses, are described below.

The cost of diodes, for a constant mean power, is significantly lower ifthese diodes operate continuously.

Furthermore, a laser device according to the invention may be muchsimpler than a laser device according to prior art because this devicecan operate without putting amplifiers in series.

The operation and maintenance of this laser device is less expensive dueto the small number of optical components used.

Greater usage flexibility is possible due to the fact that severaloscillators are put in parallel.

The increase in the number of optical resonators also makes the deviceaccording to the invention less sensitive to an incident affecting theinstantaneous performances of one of the optical resonators.

BRIEF DESCRIPTION OF THE FIGURES

This invention will be better understood after reading the followingdescription of example embodiments given purely for information andwhich in no way is limitative, with reference to the attached drawingsin which:

FIGS. 1 and 2 diagrammatically illustrate the use of two laser beamsfocused differently to locally obtain high illumination, and havealready been described,

FIG. 3 diagrammatically illustrates an example of a composite lightpulse that can be used in this invention and that has already beendescribed,

FIG. 4 is a diagrammatic view of a combination of several opticalresonators according to the invention in order to create an excitationdevice for a light source in the extreme ultraviolet,

FIG. 5 diagrammatically illustrates a particular embodiment of theoptical resonator according to the invention, and

FIGS. 6 and 7 diagrammatically and partially illustrate other examplesof the invention, enabling spatial multiplexing of elementary laserbeams generated individually by several optical resonators.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

An optical resonator conform with the invention is shown in FIG. 5, andwill be described in more detail later. It may be followed by one orseveral single pass amplifiers.

The combination of several pulsed optical resonators according to theinvention in order to create an excitation device for a light source inthe extreme ultraviolet is shown diagrammatically in FIG. 4.

The device in FIG. 4 comprises more than three pulsed opticalresonators, that are also called pulsed lasers, for example ten, butonly three of them are shown in this FIG. 4 and their reference numbersare 2, 4 and 6 respectively.

The light beams 8, 10 and 12 (more precisely the light pulses) suppliedby these pulsed optical resonators 2, 4 and 6 were sent through a set ofmirrors 14 to approximately the same point P on a target 16 and arrivingat this point P at approximately the same time.

Laser control means 18 are also shown, capable of obtaining laserpulses.

FIG. 4 also shows focusing means 20, 22 and 24, that for example areachromatic doublets designed to focus light beams 8, 10 and 12respectively on point P of the target 16.

In the example considered, the lasers and the target are chosen tooutput an EUV radiation 26 by interaction of the light beams with thistarget. In order to do this, the target includes for example anaggregate jet 28 (for example xenon) output from a nozzle 30.

For example this EUV radiation 26 may be used for microlithography of anintegrated circuit 32. The block 34 in FIG. 4 symbolises the variousoptical means used to shape the EUV radiation before it reaches theintegrated circuit 32.

Lasers 2, 4 and 6 are identical or almost identical and are capable ofsupplying light pulses.

Each laser comprises two pumping structures 36 a and 36 b, for which theaberration and birefringence are low.

The structure 36 a comprises a laser rod 38 a pumped by a set of laserdiodes 40 a, and the structure 36 b comprises a laser rod 38 b pumped bya set of laser diodes 40 b, operating continuously.

The material chosen for our experiments is Nd:YAG, for which thesaturation fluence is 200 mJ/cm²;

However, it may be advantageous to choose a different laser from theothers to create the first pulse called the prepulse.

Each optical resonator directly produces a power of 300 W at 10 kHz,with a beam quality compatible with multiplexing, the pulse durationbeing 50 ns and its energy being 300 mJ. The fluence of the beam at theexit from the cavity is 2.3 J/cm², which is almost ten times thesaturation fluence of the Nd:YAG material.

The focusing of the beam produced by each laser 2, 4 and 6 on a 50 μmdiameter area of the target then leads to a peak power of 3×10¹⁰ W/cm²to 6×10¹⁰ W/cm².

A value of 5×10¹¹ W/cm² is a typical target value to be achieved, inorder to obtain sufficient emissivity on a liquid xenon target.

Therefore, this is obtained by combining ten lasers with theperformances mentioned above.

No light amplifier is used with lasers 2, 4 and 6 in the example in FIG.4.

However, it would be possible to add such an amplifier or even severalsuch amplifiers after each optical resonator, if this is found to benecessary to adjust the peak power to an optimum determined byexperience.

Allowing for the features of the optical resonator according to theinvention, these amplifiers would operate with a relatively low gain butwith optimum extraction of the energy deposited in the rod of thisamplifier considering the fluence about 10 times greater than thesaturation fluence of the material of this rod.

FIG. 5 shows a diagrammatic view of a pulse optical resonator accordingto the invention. It is composed like any one of resonators 2, 4 and 6and thus comprises structures 36 a and 36 b and mirrors 42 and 44, thepolarisation rotator 46 and/or the lens 46 a and the means of triggeringpulses 50 and 52 that will be described later.

In one variant embodiment, a light amplifier 36 c is placed at theoutput from this optical resonator.

This amplifier 36 c comprises a single pass laser-rod 38 c pumped by aset of laser diodes 40 c operating continuously.

Control means 18 are then provided to control this amplifier 36 c. Thisamplifier is substantially identical to the structures 36 a and 36 b andits laser rod 38 c is preferably made from the same laser material asthe laser rods 38 a and 38 b.

This laser material is chosen from among Nd:YAG (the preferredmaterial), Nd:YLF, Nd:YALO, Yb:YAG, Nd:ScO₃ and Yb:Y₂O₃.

With reference once again to FIG. 4, each optical resonator is delimitedby a first highly reflecting mirror 42 (reflection coefficient R equalto 100%, for example at 1064 nm) and a second mirror 44 that ispartially reflecting (R of the order of 70% to 80%) to allow the lightbeam generated by this optical resonator to pass through it.

These mirrors are preferably curved and their radii of curvature arecalculated so that the divergence of the beam is small, and such thatthe parameter M² is equal to about 10.

Furthermore, the length of the cavity is chosen as a function of theduration of the pulses.

The two curved mirrors may be replaced by two sets each comprising adivergent lens and a plane mirror.

Preferably, identical pumping structures are used in each of the lasers2, 4 and 6 to compensate for the different thermal effects that canoccur. But in this case, it is better to use a 90° polarisation rotator46 at any location between the two laser rods 38 a and 38 b.

Instead of the rotator 46, a slightly divergent lens 46 a could be usedat exactly the mid path between the two rods.

As a variant, this lens in this arrangement and the rotator 46 could beused, the rotator still being located between the two rods adjacent tothe lens.

The diameter of these laser rods is between 3 mm and 6 mm.

We use 4 mm diameter rods made of Nd:YAG doped at 1.1% in ourexperiments.

Furthermore, in the example in FIG. 4, each Nd YAG rod is pumped by 40laser diodes, each of these diodes having a power of 30 W and emittingat 808 nm.

Each rod is preferably pumped homogeneously, in order to minimisespherical aberrations.

In order to make each laser pulsed, acousto-optic pulse triggering meansare placed in the cavity on the path of the beam, at the location atwhich it diverges least, in other words between each of the rods and thepolarisation rotator, to enable triggering of these pulses at a highrate.

Each of these acousto-optic triggers or Q-switches uses a silica crystaloperating in compression mode with a radio frequency power of 90 W at 27MHz, this power being applied on the crystal by a 4 mm transducer.

In the example in FIG. 4, two acousto-optic deflectors 50 and 52 of thetype defined above are used, and are controlled by control means 18located in the space delimited by the laser rods 38 a and 38 b on eachside of the polarisation rotator 46.

These two acousto-optic deflectors 50 and 52 are used to block thecavity with gains corresponding to the average power mentioned above.

The control means 18 trigger operation of the EUV source to adapt itscharacteristics to the needs of microlithography. If applicable, theydetermine the simultaneousness of light pulses of lasers 2, 4 and 6 atthe target.

If the optical paths have significantly different lengths, in particularthey will be capable of compensating for these differences and managingtriggering of all acousto-optic deflectors contained in the device inFIG. 4 so that synchronism is achieved for light pulses.

The control means 18 comprise:

-   -   means (not shown) of generating pumping laser diode power supply        currents 40 a and 40 b (and possibly 40 c) and    -   means (not shown) of generating modulated radio frequency        currents, to control each pair of acousto-optic deflectors 50        and 52 almost synchronously, the offset between these deflectors        preferably being less than 1 ns.

Furthermore, these control means 18 are designed to control lasers 2, 4and 6 as a function of the plasma radiation measurement signals(generated by the interaction of laser beams with the target 16),supplied by one or several appropriate sensors such as the sensor 54,for example one or several fast silicon photodiodes with spectralfiltering; for EUV radiation, this filtering may be done by zirconium,and by a molybdenum-silicon multilayer mirror, possibly doubled up; ifthe plasma growth rate is observed, either this filtering should bemodified, or one or several other fast photodiodes with filtering closerto the visible spectrum should be added.

Control means 18 are also provided to control lasers 2, 4 and 6 as afunction of:

-   -   signals for measuring the energy of light pulses from lasers 2,        4 and 6, that are provided by appropriate sensors 56, 58 and 60        respectively, for example fast silicon photodiodes with        integrating means, and    -   signals for measuring the time shapes of light pulses from        lasers 2, 4 and 6, signals that are provided by three        appropriate sensors 62, 64 and 66 respectively, for example fast        silicon photodiodes that may be the same sensors as sensors 56,        58 and 60, except that the signal is then taken off on the input        side of the integration means.

Note that the optical means composed of the deflection mirrors 14 andthe achromatic focusing doublets 20, 22 and 24 are chosen to enablespatial superposition with position fluctuations smaller than a lowpercentage, for example of the order of 1% to 10%, of the diameter ofthe focal spot (point P).

The laser device in FIG. 4 also comprises means designed to modify thespatial distribution of the pulse resulting from the addition of lightpulses emitted by lasers 2, 4 and 6 respectively. These means,symbolised by arrows 74, 76 and 78 may for example be designed todisplace achromatic doublets 20, 22 and 24, so as to modify the sizes ofthe focal spots output by each of these doublets respectively.

The control means 18 may be designed to shift the light pulses emittedby lasers 2, 4 and 6 with respect to each other in time, by shifting thetriggering of lasers with respect to each other in an appropriatemanner.

Note that the laser device in FIG. 4 is not polarised, unlike otherknown laser devices, for example as described in document [5].

Maintaining polarisation with Nd:YAG based lasers is difficult and makesthe device more complicated. However, the modular design of theinvention with spatial multiplexing means that it is not essential forthe laser device to be polarised.

If higher repetition rates are required, greater than or equal to 10kHz, it is preferable to avoid using variants with time multiplexing.Pulses derived from N lasers (for example N=10) then reach the target atexactly the same time.

One variant embodiment of the invention is diagrammatically andpartially shown in FIG. 6. In this variant, spatial multiplexing of thelaser beams 8, 10 and 12 is used before they are focused on the targetP.

This is done by replacing the last two mirrors 14 (top of FIG. 4) thatare associated with the beams 10 and 12, by two drilled mirrors 80 and82 aligned with the last mirror 14 (top of the FIG. 4) associated withbeam 8.

Thus, the drilled mirror 80 allows part of the beam 8 to pass throughthe target and reflects part of the beam 10 towards the target. A meansof stopping the beam 84 is provided to stop the rest of the beam 10 (notreflected towards the target).

Similarly, the drilled mirror 82, in which the drilling is larger thanthe drilling in the mirror 80, allows part of the beams 8 and 10 to passthrough towards the target and reflects part of the beam 12 towards thistarget. A means of stopping the beam 86 is provided to stop the rest ofthe beam 12 (not reflected towards the target).

An achromatic focusing doublet 88 is designed to focus the beams outputfrom the aligned mirrors 14, 80 and 82 onto the target.

Another variant embodiment of the invention is diagrammatically andpartially shown in FIG. 7. In this variant, the drilled mirror 80 may bereplaced by a sharp edged mirror 90 designed to reflect part of the beam8 towards this target. A means of stopping the beam 94 is provided tostop the rest of the beam 10 (not reflected towards the target).

The drilled mirror 82 is also replaced by another sharp edged mirror 92designed to reflect part of the incident beam 12 towards the target,allowing part of the beams 8 and 10 to pass at its periphery towardsthis target. A means of stopping the beam 96 is provided to stop theremainder of the beam 12 (not reflected towards the target).

Achromatic focusing doublets 20, 22, 24 and 88 are advantageouslydesigned to minimise aberrations. But they may be replaced by curvedmirrors.

1-17. (canceled)
 18. An optical resonator with a solid state amplifyingmedium, the optical resonator being pulsed and pumped by diodesoperating continuously, and comprising: at least two laser rods; atleast one means for triggering light pulses, the means for triggeringlocated in a part of the optical resonator in which a laser beamgenerated by the optical resonator diverges least; and first and secondmirrors that delimit a cavity of the optical resonator, the first mirrorbeing reflecting and the second mirror being partly reflecting.
 19. Anoptical resonator according to claim 18, wherein the at least two laserrods comprise isotropic material of Nd:YAG or Yb:YAG, and the cavitycomprises means for polarization rotation on a path of the laser beam ineach of spaces formed by two successive of the at least two rods, therotation being 90°.
 20. An optical resonator according to claim 18,further comprising a divergent lens, in a middle of each intervalbetween two adjacent rods of the at least two rods.
 21. An opticalresonator according to claim 18, wherein a laser material from which theat least two laser rods are made is chosen from the group comprisingNd:YAG, Nd:YLF, Nd:YALO, Yb:YAG, Nd:ScO₃, and Yb:Y₂O₃.
 22. An opticalresonator according to claim 18, comprising two rods made of a lasermaterial, substantially identical, and means for polarization rotationplaced in an area between the two rods.
 23. An optical resonatoraccording to claim 20, wherein the means for triggering pulses placed ineach pulsed optical resonator comprises two Q-switches located in theinterval, on each side of the means for polarization rotation, betweenthe means for polarization rotation and the at least two laser rods. 24.An optical resonator according to claim 19, wherein the means fortriggering are of acousto-optical type.
 25. An optical resonatoraccording to claim 18, associated with one or plural single passamplifiers.
 26. A laser device, comprising: at least three pulsedoptical resonators according to claim 18; and means for transferringlight pulses to substantially a same location on a target and atsubstantially a same time at the location; and means for controlling theat least three pulsed optical resonators, so that all means fortriggering forming part of the device operate synchronously.
 27. Adevice according to claim 26, comprising at least ten pulsed opticalresonators in parallel.
 28. A device according to claim 26, wherein themeans for transferring light pulses comprises means for transferring thelight pulses onto the target along a same path.
 29. A device accordingto claim 26, further comprising means for modifying a spatialdistribution of a light pulse resulting from addition of light pulsesoutput by the at least three optical resonators.
 30. A device accordingto claim 26, wherein the means for controlling the at least three pulsedoptical resonators also are for modifying a time distribution of a lightpulse resulting from addition of light pulses supplied by the at leastthree optical resonators, to create composite pulses.
 31. A deviceaccording to claim 30, wherein a profile of each composite pulsecomprises a first plasma ignition pulse created by interaction of thelight pulses with the target, wherein a time interval in which the lightenergy output by the laser is minimum during plasma growth, and whereina second pulse is composed of plural elementary pulses according to asequence that depends on plasma growth.
 32. A device according to claim26, further comprising means for modifying a recurrence rate of lightpulses emitted by the at least three optical resonators or a sequence ofthe light pulses emitted by the at least three optical resonators.
 33. Adevice according to claim 30, capable of sending a first highly focusedbeam onto the target and then applying a remainder of the light energyonto the target with broader focusing.
 34. A device according to claim26, wherein the target is configured to output light in an extremeultraviolet domain by interaction with the light pulses emitted by theat least three optical resonators.